Just in time for NaNoWriMo, Nathan Bransford offers this very helpful post on how to start writing a novel. Yes, I know: just start. Today of all days, just start. That's probably the best advice of all. But Bransford points out the two elements you need in order for the novel to take shape: voice and plot.
In particular, I can't overemphasize the importance of finding the voice--which, as Bransford says, is the novel's sensibility. (Be sure to read his post on the elements of a successful voice.) Here's my two bitcoins on the matter: Voice is close to tone and is reflected in tone, but it's more the stance toward the story. The stance is personified as some form of narrator or narrative presence, and is evident in the narrator's word choice, pace, tone--the whole stylistic kit and kaboodle. Now, you may not think there's an actual narrator in your novel, at least not akin to Thackeray's or even Austen's convivial, sardonic "I." But it's worth deciding there is always a narrator, even if he or she stays far behind the scenes, pretending she doesn't actually exist. Thinking this way allows you to distance yourself at least a tiny bit from the voice that is telling your story, which then allows you to make conscious decisions about what the narrator--again, not necessarily you--thinks and feels about what's going on. In fact, it's been my experience that a certain productive tension can result when I decide that the narrator of a particular story is going to feel somewhat different about its events than I, personally, would feel. This curtails the temptation to turn the story into a self-pity wallow or a soapbox, and it allows for unexpected experiences of empathy--which are the best kind.
Plot, for me, is even tougher to tease out--but I think that, too, has a relationship to voice. What your narrator chooses to tell, how she chooses to tell it, and why she tells it all are all aspects of plot. You have a sensibility that's picking and choosing, so understanding that sensibility is key to making those choices.
Of course, like everything else in novel writing, your voice for the novel will probably not spring fully formed from your head. It will come out in the writing itself. It may gradually assert itself more and more as the draft moves forward, or it may pop out in odd little asides, where you least expect it. The point is to realize you need it, and to keep an eye and ear out for it, as you work through your early drafts.